Parameterization of the Effects of Temperature Atmospheric Carbon Dioxide Partial Pressure and Runoff on the Weathering Rate

An understanding of the possible effect of biota on the habitability of the Earth's surface is dependent on its qualitative and quantitative involvement in the carbonate-silicate biogeochemical cycle. Two different approaches have been taken with respect to modeling the steady-state atmospheric pCO2/ surface temperature as an outcome of this cycle. One approach has been to parameterize the abiotic and biotic effects as best as can be estimated giving a BLAG or GEOCARB explicit expression of their separate and combined effects on the rate of chemical weathering on a real (biotic) Earth surface. For example, Gwiarzda and Broecker (1994) constructed such a model of current weathering in soil. They included temperature, soil pCO2 and organic acidity in their model of silicate weathering in a temperate climate, finding that temperature was the main control on weathering rate. Two of their most questionable assumptions were that dissolution occurs far from equilibrium and that microenvironmental effects such as elevation oforganic acid concentration and lowering of pH in the rhizosphere are not present. They conceded that microenvironmental effects might be very significant.

The other approach, which we have developed, is to lump all biotic effects, direct and indirect, into a biotic enhancement of weathering factor, with a separate and entirely abiotic parameterization of the effects of atmospheric pCO2/surface temperature on the chemical weathering rate (Schwartzman and Volk 1989). The original WHAK model essentially treats the kinetics ofweathering as an abiotic process with no explicit treatment of biotic effects.

It is important to keep the assumptions in the two distinct approaches in mind, for considerable confusion may result in comparing inferences from each model. For example, one should not conclude from modest incremental biotic effects on chemical weathering rates (e.g., in soil pCO^) that the absolute biotic enhancement of weathering is small. An abiotic Earth surface could be much hotter than a biotic one, with roughly similar responses to changes in surface temperature and atmospheric pCO2 levels (i.e., abiotic and biotic cases with similar sensitivities to changes in these parameters). This possibility is analogous to the response of a thermostat at two widely different temperature settings. Analogous misleading conclusions are made from the comparison of field weathering rates to model rates computed from laboratory experiments (see previous discussion in chapter 5).

The physical variables thought to be relevant to the carbonate-silicate biogeochemical cycle include mean and regional surface temperature, land area (at least as a first approximation), volcanic/metamorphic outgassing rate, atmospheric pCO2, soil pCO2, and runoff. We will now discuss the parameterization of these variables in modeling.

0 0

Post a comment